Not really a good plug for that location. How accessible is the flange right now? How will it be covered?
Since the tank will remain empty I'd clean it well with bleach and then tape some heavy plastic over the flange ensuring a good seal. I'd recommend aluminum foil duct tape since it won't dry out like duck tape. Thin piece of wood over that to protect the plastic.
You could try one of these;
Think it would need to be 3" though, check the hole in your flange, it might stick up too far if you're putting the composting toilet in the same place though.